1. Technical Field
This invention relates to cryogenic gas processes for separating multi-component gaseous hydrocarbon streams to recover both gaseous and liquid compounds. More particularly, the cryogenic gas processes of this invention utilize a high pressure absorber.
2. Background and Prior Art
In most plants, gas processing capacity is generally limited by the horsepower available for recompression of the pipeline sales gas stream. The feed gas stream is typically supplied at 700-1500 psia and expanded to a lower pressure for separation of the various hydrocarbon compounds. The methane-rich stream produced is typically supplied at about 150-450 psia and is recompressed to pipeline sales gas specifications of 1000 psia or above. This pressure difference accounts for the major portion of the horsepower requirement of a cryogenic gas processing plant. If this pressure difference can be minimized, then more recompression horsepower will be available, thereby allowing increased plant capacity of existing gas processing plants. Also, the process of the invention may offer reduced energy requirements for new plants.
Cryogenic expansion processes produce pipeline sales gas by separating the natural gas liquids from hydrocarbon feed gas streams.
In the prior art cryogenic processes, a pressurized hydrocarbon feed gas stream is separated into constituent methane, ethane (C2) compounds and/or propane (C3) compounds via a single column or a two-column cryogenic separation schemes. In single column schemes, the feed gas stream is cooled by heat exchange contact with other process streams or external refrigeration. The feed gas stream may also be expanded by isentropic expansion to a lower pressure and thereby further cooled. As the feed stream is cooled, high pressure liquids are condensed to produce a two-phase stream that is separated in one or more cold separators into a high pressure liquid stream and a methane-rich vapor stream in one or more cold separators. These streams are then expanded to the operating pressure of the column and introduced to one or more feed trays of the column to produce a bottom stream containing C2 compounds and/or C3 compounds and heavier compounds and an overhead stream containing methane and/or C2 compounds and lighter compounds. Other single column schemes for separating high pressure hydrocarbon streams are described in U.S. Pat. Nos. 5,881,569; 5,568,737; 5,555,748; 5,275,005 to Campbell et al; U.S. Pat. No. 4,966,612 to Bauer; U.S. Pat. Nos. 4,889,545; 4,869,740 to Campbell; and U.S. Pat. No. 4,251,249 to Gulsby.
Separation of a high pressure hydrocarbon gaseous feed stream may also be accomplished in a two-column separation scheme that includes an absorber column and a fractionation column that are typically operated at very slight positive pressure differential. In the two-column separation scheme for recovery of C2+ and/or C3+ natural gas liquids, the high pressure feed is cooled and separated in one or more separators to produce a high pressure vapor stream and a high pressure liquid stream. The high pressure vapor stream is expanded to the operating pressure of the fractionation column. This vapor stream is supplied to the absorber column and separated into an absorber bottom stream and an absorber overhead vapor stream containing methane and/or C2 compounds along with trace amounts of nitrogen and carbon dioxide. The high pressure liquid stream from the separators and the absorber bottom stream are supplied to a fractionation column. The fractionation column produces a fractionation column bottom stream which contains C2+ compounds and/or C3+ compounds and a fractionation column overhead stream which may be condensed and supplied to the absorber column as reflux. The fractionation column is typically operated at a slight positive pressure differential above that of the absorber column so that fractionation column overheads may flow to the absorber column. In many of the two-column systems, upsets occur that cause the fractionation column to pressure up, particularly during startup. Pressuring up of the fractionation column poses safety and environmental threats, particularly if the fractionation column is not designed to handle the higher pressure. Other two-column schemes for separating high pressure hydrocarbon streams are described in U.S. Pat. No. 6,182,469 to Campbell et al.; U.S. Pat. No. 5,799,507 to Wilkinson et at.; U.S. Pat. No. 4,895,584 to Buck et al.; U.S. Pat. No. 4,854,955 to Campbell et al.; U.S. Pat. No. 4,705,549 to Sapper; U.S. Pat. No. 4,690,702 to Paradowski et al.; U.S. Pat. No. 4,617,039 to Buck; and U.S. Pat. No. 3,675,435 to Jackson et al.
U.S. Pat. No. 4,657,571 to Gazzi discloses another two-column separation scheme for separating high pressure hydrocarbon gaseous feed streams. The Gazzi process utilizes an absorber and fractionation column that operate at higher pressures than the two-column schemes discussed above. However, the Gazzi process operates with the absorber pressure significantly greater than the fractionation column pressure, as opposed to most two-column schemes that operate at a slight pressure differential between the two vessels. Gazzi specifically teaches the use of a dephlegmator within the fractionation column to strip the feedstreams of a portion of the heavy constituents to provide a stripping liquid for use in the absorber. Gazzi""s tower operating pressures are independent of each other. The separation efficiency of the individual towers is controlled by individually altering the operating pressure of each tower. As a result of operating in this manner, the towers in the Gazzi process must operate at very high pressures in order to achieve the separation efficiency desired in each tower. The higher tower pressures require higher initial capital costs for the vessels and associated equipment since they have to be designed for higher pressures than for the present process.
It is known that the energy efficiency of the single column and two-column separation schemes may be improved by operating such columns at higher pressure, such as in the Gazzi patent. When operating pressures are increased, however, separation efficiency and liquid recovery are reduced, often to unacceptable levels. As column pressures increase, the column temperatures also increase, resulting in lower relative volatilities of the compounds in the columns. This is particularly true of the absorber column where the relative volatility of methane and gaseous impurities, such as carbon dioxide, approach unity at higher column pressure and temperature. Also, the number of theoretical stages in respective columns will have to increase in order to maintain separation efficiency. However, the impact of the residue gas compression costs prevails above other cost components. Therefore, the need exists for a separation scheme that operates at high pressures, such as pressures above about 500 psia, yet maintains high hydrocarbon recoveries at reduced horsepower consumption.
Earlier patents have addressed the problem of reduced separation efficiency and liquid recovery, typically, by introducing and/or recycling ethane-rich streams to the column. U.S. Pat. No. 5,992,175 to Yao discloses a process for improving recovery of C2+ and C3+ natural gas liquids in a single column operated at pressures of up to 700 psia. Separation efficiency is improved by introducing to the column a stripping gas rich in C2 compounds and heavier compounds. The stripping gas is obtained by expanding and heating a liquid condensate stream removed from below the lowest feed tray of the column. The two-phase stream produced is separated with the vapors being compressed and cooled and recycled to the column as a stripping gas. However, this process has unacceptable energy efficiency due to the high recompression duty that is inherent in one-column schemes.
U.S. Pat. No. 6,116,050 to Yao discloses a process for improving the separation efficiency of C3+ compounds in a two-column system, having a demethanizer column, operated at 440 psia, and a downstream fractionation column, operated at 460 psia. In this process, a portion of a fractionation column overhead stream is cooled, condensed and separated with the remaining vapor stream combined with a slip stream of pipeline gas. These streams are cooled, condensed and introduced to the demethanizer column as an overhead reflux stream to improve separation of C3 compounds. Energy efficiency is improved by condensing the overhead stream by cross exchange with a liquid condensate from a lower tray of the fractionation column. This process operates at less than 500 psia.
U.S. Pat. No. 4,596,588 to Cook discloses a process for separating a methane-containing stream in a two-column scheme, which includes a separator operating at a pressure that, is greater than that of a distillation column. Reflux to the separator may be obtained from one of the following sources: (a) compressing and cooling the distillation column overhead vapor; (b) compressing and cooling the combined two-stage separator vapor and distillation column overhead vapor; and (c) cooling a separate inlet vapor stream. This process also appears to operate at less than 500 psia.
Heretofore, there has not been a cryogenic process for separating multi-compound gaseous hydrocarbon streams to recover both gaseous and liquid compounds in one or more high pressure columns. Therefore, the need exists for a two-column scheme for separating a high pressure, multi-compound stream wherein the pressure of an absorber is substantially greater than and at a predetermined differential pressure from the pressure of a downstream fractionation column that improves energy efficiency, while maintaining separation efficiency and liquid recovery.
The present invention disclosed herein meets these and other needs. The goals of the present invention are to increase energy efficiency, provide a differential pressure between the absorber and fractionation columns, and to protect the fractionation column from rising pressure during startup of the process.
The present invention includes a process and apparatus for separating a heavy key component from an inlet gas stream containing a mixture of methane, C2 compounds, C3 compounds and heavier compounds wherein an absorber is operated at a pressure that is substantially greater than the fractionation column pressure and at a specific or predetermined differential pressure between the absorber and the fractionation column. The heavy key component can be C3 compounds and heavier compounds or C2 compounds and heavier compounds. The differential pressure in this process is about 50 psi to 350 psi between the absorber and the fractionation column.
An inlet gas stream containing a mixture of methane, C2 compounds, C3 compounds and heavier compounds is cooled, at least partially condensed and separated in a heat exchanger, a liquid expander, vapor expander, an expansion valve or combinations thereof, to produce a first vapor stream and a first liquid stream. The first liquid stream may be expanded and supplied to a fractionation column along with a fractionation feed stream and a fractionation reflux stream. These feed streams may be supplied to a middle portion of the fractionation column and warmed by heat exchange contact with residue gas, inlet gas, absorber overhead stream, absorber bottom stream and combinations thereof in an apparatus such as consisting of a heat exchanger and a condenser. The fractionation column produces a fractionation overhead vapor and a fractionation bottom stream. The first vapor stream is supplied to an absorber along with an absorber reflux stream to produce an absorber overhead stream and an absorber bottom stream.
At least a portion of the fractionation overhead stream is at least partially condensed and separated to produce a second vapor stream and the fractionation reflux stream. The second vapor stream is compressed to essentially about the absorber pressure to produce a compressed second vapor stream that is at least partially condensed by heat exchange contact with one or more process streams such as the absorber bottom stream, the absorber overhead stream, at least a portion of the first liquid stream or combinations thereof. The compressed second vapor stream contains a major portion of the methane in the fractionation feed stream and second fractionation feed stream. When the heavy key component is C3 compounds and heavier compounds, then the compressed second vapor stream additionally contains a major portion of the C2 compounds in the fractionation feed stream and second fractionation feed stream. This stream is then supplied to the absorber as an absorber feed stream. The absorber overhead stream may be removed as a residue gas stream containing substantially all of the methane and/or C2 compounds and a minor portion of C3 or C2 compounds. Such residue gas stream is then compressed to pipeline specifications of above about 800 psia. The fractionation bottom stream can be removed as a product stream containing substantially all of the C3 compounds and heavier compounds and a minor portion of the methane and C2 compounds.
In this invention, the absorber pressure is above about 500 psia. The apparatus for separating the heavy key component from an inlet gas stream containing a mixture of methane, C2 compounds, C3 compounds and heavier compounds, includes a cooling means. When the heavy key component is C3 compounds and heavier compounds, an apparatus for separating the heavy key component from an inlet gas stream comprises a cooling means for at least partially condensing the inlet gas stream to produce a first vapor stream and a first liquid stream; a fractionation column for receiving the first liquid stream, a fractionation feed stream and a second fractionation feed stream, the fractionation column produces a fractionation bottom stream and a fractionation overhead vapor stream; a condenser for at least partially condensing the overhead vapor stream to produce a second vapor stream and a fractionation reflux stream; an absorber for receiving at least a portion of the first vapor stream and an absorber feed stream, the absorber produces an absorber overhead stream and a second fractionation feed stream, the absorber having a pressure that is substantially greater than and at a predetermined differential pressure from the fractionation column pressure; a compressor for compressing the second vapor stream essentially to absorber pressure to produce a compressed second vapor stream; a condensing means for at least partially condensing the compressed second vapor stream to produce the absorber feed stream; and whereby the fractionation bottom stream contains a majority of heavy key components and heavies.